Method of forming a modular heat exchanger having a brazed core

ABSTRACT

A modular heat exchanger suitable for automotive applications, and methods for forming the modular heat exchanger. The modular heat exchanger construction incorporates a monolithic brazed core assembly composed of flat-type cooling tubes and sinusoidal centers. The required positional tolerances of the tubes for mating with the remainder of the heat exchanger are maintained within the brazed core assembly by minimizing core shrinkage resulting from the brazing operation during which the tubes are brazed to the centers.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division patent application of co-pending U.S. patentapplication Ser. No. 08/489,795, filed Jun. 13, 1995, whose contents areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an improved modular heat exchanger ofthe type used in the automotive industry. More particularly, thisinvention relates to a modular heat exchanger having a brazed coreassembly and techniques by which such a heat exchanger is assembled, inwhich the brazed core is manufactured such that the spacial positions ofits cooling tubes after brazing enable the tubes to properly mate withelastomerically-sealed apertures formed in the heat exchanger's headers,and thereby form a modular heat exchanger whose brazed core can bereadily removed for repair or replacement, and whose overallconstruction is capable of withstanding structural distortions withoutleaks and structural failures occurring between the tubes and theheaders.

Heat exchangers are routinely employed within the automotive industry,such as in the form of radiators for cooling engine coolant, oilcoolers, charge air coolers, condensers and evaporators for airconditioning systems, and heaters. In order to efficiently maximize theamount of surface area available for transferring heat between theenvironment and a fluid flowing through the heat exchanger, the designof the heat exchanger is often of a brazed tube-and-fin type, such asthe tube-and-center construction illustrated in FIG. 1 a. A portion of aheat exchanger 10 is shown as having a number of flat-type cooling tubes12 between pairs of high surface area fins, or centers 14. The centers14 enhance the ability of the heat exchanger 10 to transfer heat fromthe fluid flowing through the tubes 12 to the environment, or vice versaas the case may be. The tubes 12 are simultaneously brazed to thecenters 14 and a manifold (not shown) to form a monolithic brazedconstruction defining a fluid circuit.

For reasons of weight and durability, the automotive heat exchangerindustry has gradually converted to an aluminum alloy construction. Thetubes and centers of such heat exchangers are conventionally formed froman aluminum alloy that is clad with an aluminum-silicon eutectic brazingalloy, such as AA 4045, AA 4047 and AA 4343 aluminum alloys (AA beingthe designation given by the Aluminum Association), or another suitablebrazing alloy, including zinc-base cladding alloys. Such braze alloyshave a lower melting temperature than the base aluminum alloy, which isoften AA 3003, having a nominal chemistry of about 1.2 weight percentmanganese, with the balance being substantially aluminum. The claddingis formed to provide a sufficient amount of braze alloy to producefluid-tight brazements when the assembled components are heated to atemperature above the melting temperature of the cladding, but below themelting temperature of the base aluminum alloy. Aluminum heat exchangertubes are generally seamless extruded tubes, preformed welded tubes, orformed from welded strips of aluminum flat stock, with one or both sidesof the tube being clad.

While the above-described brazed tube-and-center construction is widelyemployed in the automotive industry for the manufacture of heatexchangers, certain disadvantages exist. One drawback is that themonolithic brazed construction requires a large drying oven and furnace,both of which are expensive to purchase and operate. Another drawback isthat the brazed construction renders such heat exchangers inadequate formore physically demanding applications, such as in the truck andheavy-duty equipment industries. More specifically, monolithic brazedheat exchangers are rigid and therefore do not readily “give” duringpressure and thermal cycles, when subject to vibration, or whenotherwise distorted by their operating environment. For example,radiators used in large trucks and other large equipment typically haveframe mountings that tend to distort the radiator into a parallelogramshape when the vehicle is moving over an uneven surface and whensufficient engine torque is generated. As a result, if a monolithicbrazed heat exchanger is used in these applications, cracks eventuallydevelop in the tube-to-header joint where the tubes are brazed to themanifold. Repair of cracks in the tube-to-header joint is expensive, andthis mode of failure constitutes a major source of scrappage in the heatexchanger industry. Finally, the working environment of a heavy-dutyvehicle employed in construction is severe, leading to a high incidenceof damage to the tubes and fins from impacts by debris. Consequently,any localized damage to the core of a monolithic brazed heat exchangerwill generally necessitate the removal of the entire heat exchanger forrepair or replacement.

The above shortcomings are generally known in the prior art. Theresponse in the heavy duty truck and equipment industries has been toemploy a modular heat exchanger construction, such as those representedby U.S. Pat. No. 4,191,244 to Keske, U.S. Pat. No. 4,741,392 to Morse,and U.S. Pat. Nos. 5,289,870 and 5,303,770 to Dierbeck. Such designsemploy a modular radiator construction composed of a core and headerpermanently attached to a manifold or tank. One or more of theseself-contained heat exchanger units are then assembled to a commonheader or tank with the use of grommets, gaskets or other resilientsealing material to form a fluid-tight seal between the module and thecommon header or tank. Notable examples of grommet designs are taught inU.S. Pat. Nos. 4,756,361, 5,205,354 and 5,226,235 to Lesage, commonlyassigned with the present invention.

Prior art modular constructions of the type noted above have found wideuse because they are more durable and permit replacement of a damagedheat exchanger module without requiring replacement of the entire heatexchanger assembly. However, prior art modular constructions have beengenerally unable to adopt the brazed tube-and-center core designdescribed previously, in which the tubes and/or fins are formed from aclad aluminum alloy. During brazing, the core module—composed of thetubes and centers—shrinks as the cladding melts and later resolidifies.This tendency is exacerbated if sinusoidal centers are used, such asthose shown in FIG. 1 a. As a result, assembling a header or tank to thebrazed core is impossible because the tubes are misaligned withapertures formed in the header or tank to receive the tubes.

Consequently, prior art modular heat exchanger designs have beenassembled with one or more self-contained heat exchanger modules eachformed with a common inlet and outlet, as typified by Keske, Morse andDierbeck. While such an approach may solve the misalignment problemnoted above, the result is a module that is nearly as expensive tomanufacture as a conventional monolithic brazed heat exchanger. As amore economical alternative, prior art modular heat exchanges haveemployed cores with a tube and fin construction such as that illustratedin FIG. 1 b. With such designs, round tubes 16 and flat fins 18 aremechanically joined together, such as by expanding the tubes 16, astypified by the patents to Lesage. Unfortunately, mechanical joiningmethods do not yield a metal-to-metal joint that conducts heat as wellas the brazed joints formed with the brazed core design illustrated inFIG. 1 a. As a result, heat transfer between the fluid in the tubes 16and air flowing over the fins 18 is not as efficient in prior artmodular constructions as compared to monolithic brazed constructions.

From the above, it is apparent that the prior art is lacking aneconomical modular heat exchanger design that incorporates a brazed coreassembly, and further lacks a method or process by which such a heatexchanger could be manufactured. Yet it is also apparent that it wouldbe desirable to provide a heat exchanger that offers the weight anddurability benefits of an aluminum alloy construction, the heat transferefficiency of a brazed tube-and-center core construction, and aneconomical modular construction that enables a heat exchanger core to beremoved and replaced as necessary. Such a modular construction wouldpreferably incorporate a flexible seal between the heat exchanger coreand manifold, enabling distortions to occur in the heat exchangerwithout the occurrence of failures at the tube-to-header joint.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide a modular heat exchangercomposed of one or more brazed core assemblies, such that the heatexchanger is characterized by a greater heat transfer efficiency thanprior art modular heat exchangers having one or more cores constructedof mechanically joined tubes and fins.

It is a further object of this invention to provide a method thatenables the manufacture of a brazed core whose dimensions remain stableduring brazing or are substantially re-established after brazing so asto permit assembly of the core with one or more manifolds havingapertures formed therein for receiving the tubes of the brazed core.

It is another object of this invention that such a method employs aresilient tube-to-header seal that permits distortion of the heatexchanger, thereby yielding a more durable construction.

According to a preferred embodiment of this invention, these and otherobjects and advantages are accomplished as follows.

According to the present invention, an improved modular heat exchangeris provided, and particularly a modular heat exchanger of the typeemployed in trucks and other heavy-duty equipment. More specifically,the modular heat exchanger of this invention is composed of one or morebrazed cores, so as to be characterized by a high heat transferefficiency as compared to prior art modular heat exchangers formed withmechanically-joined tubes and fins. The invention further entailsseveral alternative methods by which the occurrence of core shrinkage isaverted during the brazing operation, which would otherwise prevent theassembly of the brazed core with a heat exchanger manifold. In oneembodiment, the invention utilizes a clad slurry composition thateliminates the requirement for clad tubes and fins, and therefore avertsthe occurrence of core shrinkage during brazing caused when the claddingmelts. In other embodiments, the braze operation is conducted withspacing members that physically immobilize the ends of the coolingtubes, or with a brazing fixture that enables the desired spacialpositioning of the tubes to be re-established at the completion of thebraze operation. As a result, a brazed core can be installed and removedfrom the heat exchanger as a module, thereby greatly enhancing theserviceability of the heat exchanger. Furthermore, the method preferablyemploys resilient grommets that form a flexible tube-to-header joint,permitting the heat exchanger to withstand a significant degree ofdistortion without damaging the heat exchanger or creating leaks at thetube-to-header joint.

According to one assembly method of this invention, a core is assembledwith unclad aluminum alloy tubes separated by unclad aluminum alloycenters, in which the tubes are spaced apart so as to establish a tubespacing pattern. Also formed is a header member, which may be a discretemember that is brazed, welded or otherwise an attached part of amanifold or tank assembly, or an integral portion of a manifold or tank.The header member is formed to include apertures that are spaced apartso as to correspond to the tube spacing pattern of the core. As aresult, the tubes are assemblable with the apertures upon mating of thecore with the header member. A clad slurry composition is then depositedon the core, and the core is brazed such that core shrinkage issufficiently suppressed so as to enable assembly of the tubes with theapertures of the header member. Prior to assembly with the core,grommets are provided in each aperture of the header member, such thatduring assembly, an end of each tube is received in a corresponding oneof the apertures, and the grommets form a seal between the tubes and theheader member. The above steps yield a modular heat exchanger assemblyin which the core forms a readily removable module of the heatexchanger.

According to another assembly method of this invention, a core isassembled with aluminum alloy tubes separated by aluminum alloy centers,either or both of which are clad, in which the centers may be welded totheir respective tubes. The tubes and centers are then fixtured so as toestablish a tube spacing pattern prior to brazing. A header member, asdefined above, is also formed having apertures that are spaced apart soas to correspond to the tube spacing pattern of the fixtured core. Thefixture employed is expandable, thereby allowing the core to expand dueto thermal expansion, and later shrink during cooling to enable the tubespacing to be re-established. Again, the above steps yield a modularheat exchanger assembly in which the core forms a removable module ofthe heat exchanger.

Yet another assembly method of this invention is to assemble a core withaluminum alloy tubes and aluminum alloy centers, either or both of whichare clad, and one or more spacing members that serve to establish andmaintain a desired tube spacing pattern throughout the brazingoperation. The result is again a modular heat exchanger assembly inwhich the core forms a removable module of a heat exchanger.

As can be appreciated from the above, an advantage of the presentinvention is that practicable methods are provided for manufacturing amodular heat exchanger that utilizes a brazed core construction. As aresult, a heat exchanger is provided that incorporates the advantages ofboth a modular design and a brazed core construction. For example, thebrazed core construction yields an enhanced heat transfer efficiencyattributable to a brazed tube-to-fin joint. Consequently, a heatexchanger manufactured according to this invention can be employed inplace of a larger prior art heat exchanger employing mechanically-joinedtubes and fins. In addition, brazed cores manufactured according to thisinvention require smaller drying ovens and brazing furnaces thanmonolithic brazed heat exchangers of the prior art, since the brazedcore is significantly smaller than the assembled heat exchanger unit.

Advantages of this invention associated with its modular constructioninclude a heat exchanger that is more durable for heavy-dutyapplications than heat exchangers having a monolithic brazedconstruction. For example, the use of a modular brazed core whoseindividual tubes are resiliently mounted to and sealed with the headermembers of a heat exchanger permits greater flexing without creating aleak path at the tube-to-header joint. Furthermore, the modularconstruction enables replacement of a damaged brazed core withoutnecessitating replacement or scrappage of the entire unit, such thatdowntime for the equipment is significantly reduced. Instead, the unitcan be readily dismantled, rebuilt and returned to service when needed.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of this invention will become moreapparent from the following description taken in conjunction with theaccompanying drawings, in which:

FIGS. 1 a and 1 b represent portions of prior art heat exchanger coreshave a brazed tube-and-center construction and a mechanically-joinedtube-and-fin construction, respectively;

FIG. 2 is a perspective exploded view of a modular heat exchanger havinga brazed core and sub-header assembly in accordance with one embodimentof this invention;

FIG. 3 is a plan view of the header-to-tube joint of the modular heatexchanger shown in FIG. 2;

FIGS. 4 through 6 show in cross-section three embodiments for theheader-to-tube joint of FIG. 3;

FIG. 7 represents a spring-biased core fixture used in accordance with asecond embodiment of this invention and capable of re-establishing adesired tube spacing after brazing;

FIGS. 8 a and 8 b represent the installation of a modular heat exchangerin accordance with the present invention; and

FIG. 9 is a perspective exploded view of a modular heat exchanger havinga brazed core in accordance with a second embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

An improved modular heat exchanger is provided that is suitable forautomotive applications, and particularly radiators for heavy dutyequipment. FIG. 2 represents a heat exchanger 20 that incorporates theteachings of this invention, and includes a brazed core assembly 22preferably composed of flat-type cooling tubes 24 and sinusoidal centers26. The inclusion of a brazed core assembly 22 in the heat exchanger 20of this invention represents a significant improvement over prior artmodular heat exchangers because of enhanced heat transfer efficienciesassociated with the use of flat tubes 24, sinusoidal centers 26, and atube-to-center braze joint. While prior art modular heat exchangers haveoften required the use of mechanically-joined round tubes and fins inorder to maintain positional tolerances for the tubes, the brazed coreassembly 22 of this invention is brazed in a manner that avoids coreshrinkage and therefore maintains the required positional tolerances formating the core assembly 22 with the remainder of the heat exchanger 20.

The core assembly 22 shown in FIG. 2 generally has a similarconstruction to that shown in FIG. 1 a. Air is forced around the highsurface area provided by the centers 26 located between adjacent pairsof tubes 24, providing for a high heat exchange rate between the air anda fluid flowing through the tubes 24. The tubes 24 are oriented to begeometrically in parallel with each other as well as hydraulically inparallel between a pair of manifolds 30. As shown, the tubes 24 have aflat cross-sectional shape that promotes heat transfer from the coolantto the centers 26. The tube-and-center design illustrated in FIG. 2 ispreferred because the design maximizes the amount of surface area thatis in contact with incoming air. The tubes 24 can be any one of severalknown configurations, including welded or extruded, with or without aninternal turbulator or internal fins. The tubes 24 could also be roundinstead of the flat-type configuration shown. Furthermore, the centers26 could be louvered in combination with the sinusoidal configurationshown in the Figures. A preferred embodiment employs sinusoidal centers26 with a large bend radius, preferably on the order of about 0.1 toabout 0.5 of the center's height (peak to opposite peak of adjacentbends).

The manifold design illustrated in FIG. 2 is merely one of numerousdesigns that could be employed within the teachings of this invention.As used herein, a manifold includes a header that may be a discretemember brazed or welded to the remainder of the manifold assembly, asshown in FIG. 2 in which the manifold 30 is constructed of a tank and aheader plate 40. Alternatively, the header could be mechanicallyfastened to the manifold assembly (as shown in FIG. 9), or anintegrally-formed portion of a monolithic manifold design, as is alsoknown in the art.

As illustrated in FIG. 2, the remaining components of the heat exchanger20 include upper and lower sub-headers 28, side supports 32 and grommets36. In accordance with one embodiment of this invention, the heatexchanger 20 requires the sub-headers 28 to be an integral brazed partof the core 22 in order to achieve proper alignment of the tubes 24, aswill be discussed further below. The side supports 32 are preferablyassembled to the manifolds 30 with fasteners (not shown) for the purposeof securing the manifolds 30 to the core assembly 22, yielding astructure that can be readily disassembled for repair or replacement ofthe core assembly 22.

FIG. 3 is a plan view of the header side of one of the manifolds 30shown in FIG. 2, in which the header portion, here shown as a headerplate 40, is visible through a cut-away of the grommet 36. Accordingly,the illustrated portion of the manifold 30 faces the core assembly 22,and the header plate 40 includes apertures or slots 34 for receiving theends of the tubes 24 of the core assembly 22. The slots 34 arepreferably punched into the headers plates 40 prior to their assemblywith their corresponding tank members to form the manifolds 30. Aparticularly suitable method for carrying out the above is taught inU.S. Pat. No. 5,226,235 to Lesage. As shown in FIGS. 4 through 6, theslots 34 also accommodate projecting portions of the grommets 36. As isknown in the art, the thickness of the header plates 40 and the size ofthe slots 34 must be capable of sufficiently compressing the grommets 36while minimizing the likelihood of cutting the grommets 36 in order toyield a leak-free tube-to-header seal.

In order to form a resilient and durable seal between the tubes 24 andthe header plates 40, the grommets 36 are preferably formed from anelastomeric material, such as silicone, natural rubber, or VITON, atrademark for fluoroelastomers available from Du Pont de Nemours, E.I. &Company. Openings 42 are formed in the grommets 36 to receive each ofthe tubes 24 of the core assembly 22. In conjunction with the slots 34in the header plates 40, the openings 42 must also be sized to achieveadequate compression to form a leak-free tube-to-header seal. As shownin FIGS. 2 and 3, each grommet 36 can be formed as a unitary sheet orstrip that completely overlays its respective header plate 40.Alternatively, each slot 34 of the header plates 40 can be equipped withan individual grommet. FIG. 4 represents in cross-section the appearanceof the header plate 40 and the strip grommet 36 shown in FIG. 4. FIG. 5illustrates an alternative embodiment for the grommet 36 in which rigidreinforcements 44, such as a metal or plastic rings, are in-molded inthe grommet 36 to enhance its high-pressure sealing capability.

The header plates 40 illustrated in the Figures will generally have atypical thickness of about three millimeters (about 0.125 inch). Forless demanding applications, the header plates 40 could be formed from amuch thinner material than that illustrated—for example, on the order ofabout 0.5 to about 1.5 millimeters (about 0.025 to about 0.060 inch).The slot 34 for such a header plate would preferably be formed by adrawing operation so that an annular collar is formed having a heightapproximately equal to the thickness of the header plates 40 shown inthe Figures, so as to achieve adequate compression as well as minimizethe likelihood of cutting the grommets 36. Such an approach not onlyreduces the weight of the modular heat exchanger 20 of this invention,but also permits re-coring of existing radiators with the brazed coreassembly 22 of this invention.

FIG. 6 represents the appearance of the header plate 40 and grommet 36in combination with the sub-header 28 shown in FIG. 2. As noted above,one embodiment for the heat exchanger 20 of this invention requires thesub-headers 28 in order to achieve proper alignment of the tubes 24. Asshown in FIGS. 2 and 6, the sub-headers 28 are also equipped with slots38 to receive the ends of the tubes 24, in conjunction with the openings42 in the grommets 36 and the slots 34 in the header plates 40.According to this invention, the sub-headers 28 are installed as part ofthe core assembly prior to brazing, and become an integral part of thebrazed core assembly 22. As a result, the sub-headers 28 serve as apermanent and integral alignment fixture during the braze cycle, andtherefore achieve and maintain a precise tube spacing throughout thebrazing operation employed to produce the brazed core assembly 22, eventhough core shrinkage will occur due to melting of a cladding layer onthe tubes 24 and/or centers 26.

An additional benefit is that the sub-headers 28 can be employed toattach the brazed core assembly 22 to the manifolds 30 for high pressureapplications, such as oil coolers. As shown in FIG. 6, a portion of eachgrommet 36 is sandwiched between the header plate 40 and the sub-header28. By securing the sub-header 28 to the header plate 40, the sub-header28 serves as a rigid backup that reduces the likelihood of the grommet36 rupturing or extruding when subject to high pressures. An alternativeto the sub-headers 28 is to employ individual bars (not shown), such ascarbon spacers installed between the ends of adjacent tubes 24 toachieve and maintain a precise tube spacing during the brazingoperation, and then later removed after brazing.

According to this invention, the sub-headers 28 restrict relativemovement of the ends of the tubes 24, such that the tube spacing for thebrazed core assembly 22 will correspond to that of the header plates 40,even if clad tubes 24 and/or centers 26 are used in the construction ofthe core assembly 22. Because only the ends of the tubes 24 arerestricted, the brazed core assembly 22 formed with clad tubes 24 orcenters 26 will acquire a measurable though generally unnoticeable hourglass shape when the body of the core assembly 22 shrinks as thecladding on the tubes 24 and/or centers 26 melts. Surprisingly,unimpeded shrinkage of the remainder of the core assembly 22 hasnegligible effect on tube spacing at the tube ends, such that assemblyof the brazed core assembly 22 with the manifolds 30 has been possibleunder large-scale manufacturing conditions.

While the sub-headers 28 (or other suitable spacing bars or members) arenecessary to achieve and maintain proper tube spacing in accordance withthe above-described embodiment of this invention, the sub-headers 28 arenot necessary with other manufacturing methods of this invention. Onesuch method entails the use of a spring-biased fixture 46, shown in FIG.7, to fixture the core assembly 22 prior to brazing. The fixture 46 iscomposed of a pair of movable members 48 biased toward each other withsprings 50 or other suitable biasing or load-generating devices, whichpermit the movable portion 48 of the fixture 46 to be displaced relativeto a fixed main frame 52 of the fixture 46. The ability of the movablemembers 48 to be displaced within the fixture 46 permits the coreassembly 22, when fixtured between the movable members 48, to be stackedin an oversize condition and then expand during the brazing operation asa result of thermal expansion of the tubes 24 and centers 26. Forexample, the centers 26 can be made slightly oversize (e.g., 25 to 50micrometers additional fin height) to compensate for clad melting andfin softening during brazing. The fixture 46 is able to accommodate theoversized condition of the core assembly 22, and maintain pressure onthe assembly 22 throughout the braze cycle to assure sufficient tube tocenter contact. Because the core assembly 22 is permitted to expand, thetubes 24 and centers 26 are not compressed together as they thermallyexpand, and therefore have a substantially reduced tendency forshrinkage during the braze operation as the cladding melts on the tubes24 and/or centers 26. If the sub-headers 28 are part of the coreassembly 22, as shown in FIG. 2, the core assembly 22 is furthercompressed by the fixture 46 to allow installation of the manifolds 30,resulting in a slightly convex shape to the core assembly 22. Notably,the approach of this embodiment is contrary to prior art practices,which employ rigid fixtures in order to securely hold the tubes 24 andcenters 26 in position throughout the braze cycle.

The manufacturing process for the above-described embodiments of thepresent invention is generally as follows. The core assembly 22 can beproduced simultaneously with the fabrication or forming of the manifolds30. The modular heat exchanger 20 of this invention preferably has analuminum alloy construction to yield a lower weight and more durableunit, though it is foreseeable that the manifolds 30 could be formedfrom other suitable materials, such as steel or brass. The aluminumalloy stock used to form the tubes 24 and/or centers 26 of the coreassembly 22 can be clad with any suitable braze alloy, as is theconventional practice. The core assembly 22 is assembled by stacking thetubes 24 and centers 26 and then, in accordance with the firstembodiment of this invention, fixturing the assembly with thesub-headers 28 for brazing, or in accordance with the second embodimentof this invention, fixturing the assembly with the spring-biased fixture46 shown in FIG. 7.

According to the above embodiments of this invention, brazements betweenthe tubes 24 and centers 26 are formed by the cladding on the tubes 24and/or centers 26. As is conventional, a flux compound must be depositedon the fixtured assembly in order to promote a high quality brazement. Apreferred flux compound for carrying out the teachings of this inventioncontains potassium tetrafluoroaluminate (KAIF₄), though other fluxcompounds could foreseeably be used. Prior to brazing, the fixtured andfluxed core assembly 22 will generally require drying at a temperatureof about 290° C. to about 315° C. (about 550° F. to about 600° F.) toeliminate any moisture on the surfaces of the assembly 22. Notably, thedrying operation can be carried out in a significantly smaller dryingoven than those required to dry a monolithic brazed aluminum heatexchanger. The core assembly 22 is then brazed in a controlledatmosphere. A preferred braze atmosphere is nitrogen with a free oxygenlevel of less than about 100 ppm and a dewpoint of about −40° C. Brazingis preferably carried out at a temperature of about 607° C. to about615° C. (about 1125° F. to about 1140° F.), which is sufficient to causethe cladding on the tubes 24 and/or centers 26 to melt and join eachtube 24 to its adjacent centers 26, and each of the tubes 24 to thesub-headers 28 (if present).

Upon cooling to ambient, the result is a monolithic core assembly 22that can be assembled to the remaining components of the modular heatexchanger 20. In accordance with the first embodiment of this invention,assembly is made possible because the sub-headers 28 maintain therequired tube spacing at the ends of the tubes 24. In accordance withthe second embodiment, assembly is made possible because thespring-biased fixture 46 enables the core assembly 22 to freely expandin response to thermal expansion caused by the elevated temperatures ofthe brazing cycle, such that the tubes 24 and centers 26 are notexcessively compacted during the brazing cycle. The fixture 46 alsomaintains adequate contact between the tubes 24 and centers 26throughout the braze cycle as the cladding melts, such that high qualitybrazements are achieved between the tubes 24 and centers 26. Bothmethods enable the desired tube spacing to be present after the coreassembly 22 is cooled to room temperature.

The manifolds 30 also preferable have an aluminum alloy construction,and can be fabricated concurrently with the brazing of the core assembly22. As previously discussed, the manifolds 30 may be fabricated bywelding or brazing the header plates 40 to their respective tank membersin any suitable manner. Alternatively, single piece manifolds, eachhaving an integral header portion, can be formed in accordance withknown practices. Following fabrication or forming of the manifolds 30,the grommets 36 are inserted into the slots 34 of the header plates 40,as shown in FIG. 3. Afterwards, the manifolds 30 are assembled with thebrazed core assembly 22, such that each tube 24 is inserted through agrommet opening 42. During assembly, the sub-headers 28 (if present)become positioned next to the grommets 36, as shown in FIG. 6. The finalassembly step is to attach the side supports 32 to the assembly, therebysecuring the manifolds 30 to the core assembly 22. The modular heatexchanger 20 can than be tested to ensure a leak-free construction.

In contrast to the embodiments described above, a final embodiment ofthis invention uses unclad tubes 24 and centers 26 as a solution to thecore shrinkage problem addressed by this invention. As before, the coreassembly 22 can be produced simultaneously with the fabrication of themanifolds 30. In addition, the modular heat exchanger 20 of thisembodiment preferably has an aluminum alloy construction to yield alower weight and more durable unit. However, in contrast to the previousembodiments, the aluminum alloy stock used to form the core assembly 22is not clad with a braze alloy, as is the conventional practice. As aresult, the core assembly 22 is not susceptible to shrinkage during thebrazing operation, such that the positions of the tubes 24 within thecore assembly 22 are substantially maintained throughout brazing,enabling assembly of the brazed core assembly 22 with the manifolds 30.

As with the previous embodiments, the core assembly 22 is assembled bystacking the tubes 24 and centers 26, and then fixturing the assemblyfor brazing, with or without the sub-headers 28. According to thisembodiment, brazements between the tubes 24 and centers 26 are formed bydepositing a clad slurry that contains a braze alloy powder necessary tomelt and form the brazements, and preferably a flux compound necessaryto promote a high quality brazement. According to this invention, it hasbeen determined that the use of a braze alloy powder deposited on thecore assembly 22 prior to brazing, in lieu of forming the tubes 24 andcenters 26 from conventional clad aluminum alloy stock, essentiallyeliminates core shrinkage during the brazing operation. Consequently,the dimensional spacing established for the tubes 24 prior to brazing ismaintained, thereby allowing the brazed core assembly 22 to be assembledwith the grommets 36 and manifolds boxes 30.

A preferred clad slurry for carrying out this embodiment is aflux-brazing composition taught in U.S. Pat. No. 5,251,374 to Halsteadet al. The flux-brazing composition is composed of a mixture of a fluxcompound, an aluminum-silicon braze alloy powder and a zinc powder heldtogether with a suitable binder. Preferably, the flux compound ispotassium tetrafluoroaluminate (KAIF₄). In accordance with Halstead etal., a preferred formulation for the flux-brazing composition containsabout 10 to about 50 weight percent of the potassiumtetrafluoroaluminate, about 2 to about 13 weight percent silicon, about0.5 to about 3 weight percent of zinc or another metal whose electrodepotential is less than the electrode potential of the tube's aluminumalloy, and about 0.1 to about 2 weight percent of an organic binder,such as hydroxypropyl cellulose binder, with the balance being aluminumparticles. The preferred flux-brazing composition is characterized byhaving a paste-like consistency, so as to be sufficiently viscous toadhere well to the tubes 24 and centers 26, while also beingsufficiently fluid so as to promote handling and deposition of thecomposition. Notably, the flux-brazing composition does not containwater, and therefore does not serve as a source for water that wouldotherwise adversely affect braze joint quality.

Alternative clad slurries to the preferred flux-brazing compositiontaught by Halstead et al. could be used. For example, it is foreseeablethat a clad slurry composed primarily of a braze alloy powder could beemployed under certain circumstances. Furthermore, the braze alloypowder could be other than an aluminum-silicon mixture. For example, itis foreseeable that a zinc-base powder could be employed as a substitutefor zinc cladding alloys previously noted. Such alternative compositionsare all within the scope of this invention, since the problem solved bythis invention can be characterized as the core shrinkage associatedwith brazed core constructions, and the solution provided by thisembodiment can be characterized as the identification of cladding as thesource for core shrinkage and the elimination of cladding through theuse of a braze alloy powder after the core assembly 22 has beenassembled but before the brazing operation.

Prior to brazing, the fixtured core assembly 22 must be dried at atemperature of about 260° C. to about 315° C. (about 500° F. to about600° F.) to eliminate any moisture on the surfaces of the assembly 22.The drying operation causes the preferred flux-brazing composition toharden and form a highly adherent coating that permits the core assembly22 to be readily handled without concern for the loss of flux prior tobrazing. As noted with the previous embodiments of this invention, thedrying operation can be carried out in a significantly smaller dryingoven than those required to dry a monolithic brazed aluminum heatexchanger. In addition, the preferred flux-brazing compositioneliminates the conventional requirement for a separate dry oven and corefluxer, which would require large, energy intensive facilities in orderto accommodate the core assembly 22. Instead, the preferred flux-brazingcomposition provides both the flux and the braze alloy required forbrazing.

The core assembly 22 is then brazed in a controlled atmosphere, inessentially an identical manner to that described for the previousembodiments. Brazing is preferably carried out at a temperature of about580° C. to about 620° C., which is sufficient to cause the silicon-richalloy in the flux-brazing composition to melt and join each tube 24 toits adjacent pair of centers 26, and each of the tubes 24 to thesub-headers 28 (if present). Upon cooling to ambient, the result is amonolithic brazed core assembly 22 that can be assembled to theremaining components of the modular heat exchanger 20. As with theprevious embodiments, the manifolds 30 can be fabricated or formedconcurrently with the brazing of the core assembly 22. Afterwards, themanifolds 30 can be assembled with their grommets 36 to achieve theconfiguration shown in either FIG. 4, 5 or 6, and then assembled withthe brazed core assembly 22 and the side supports 32.

From the above, it can be seen that the assembly and brazing methods ofthis invention solve the shrinkage problem that has previously preventedthe use of a brazed core assembly in a modular heat exchanger design.Specifically, this invention enables the novel construction of a modularheat exchanger with a brazed core assembly and grommets, such that theheat exchanger can be readily disassembled to remove, repair and/orreplace the brazed core assembly. Such a capability further enablesseveral small brazed cores to be assembled to a pair of manifolds asshown in FIGS. 8 a, 8 b and 9. As a result of the smaller coreassemblies required to be dried and brazed, smaller drying ovens andbraze furnaces can be used, corresponding to lower purchase andoperating costs. Because core shrinkage is eliminated or controlled bythis invention, the cores can be assembled to fit closely together,giving the appearance of a unitary core assembly.

As illustrated in FIG. 8 a, the installation of a modular heat exchangerassembly 120 in a truck or other heavy-duty vehicle typically involvesthe use of tie rods 124 that stabilize and secure the heat exchangerassembly 120 relative to the firewall 126 of the vehicle. During thevehicle's operation, the tie rods 124 restrict movement of the top ofthe assembly 120, while the bottom of the assembly 120 is subject todistortion as a result of engine torque, etc., as represented in FIG. 8b. With the use of two or more brazed core modules 122 of the type madepossible by this invention, distortion of the heat exchanger assembly120 does not cause the tube-to-header joints to fracture, but insteadpermits slippage between the core modules 122, with the grommets 36maintaining a high-integrity seal between the core modules 122 and themanifolds 130. In effect, the slippage between the core modules 122 actsas a stress reliever for the heat exchanger assembly 120, thusincreasing its service life.

A variation of the heat exchanger assembly 120 shown in FIGS. 8 a and 8b would be to use each core module 122 as a separate heat exchangercircuit, but within the single envelope formed by the manifolds 130.Such a capability with a brazed monolithic heat exchanger assembly hasnot been previously possible, because different operating temperaturesfor the individual cooling circuits would cause different thermalexpansions, leading to stress and/or cyclic failure of the heatexchanger assembly. With the present invention's brazed core assemblywithin a modular heat exchanger design, different thermal expansions cannow be accommodated by the grommets 36.

The modification represented in FIG. 9 illustrates how two or more coreassemblies 22 can be stacked on top of each other, with an intermediatemanifold 30 a being used to separate the adjacent core assemblies 22.This design allows the tubes 24 of the core assemblies 22 to expand inthe intermediate manifold 30 a during temperature cycling, therebyreducing stress and promoting a longer service life for the modular heatexchanger assembly. This modular design also solves the prior artproblem of expensive tooling and equipment to manufacture a large heatexchanger assembly. In addition, FIG. 9 illustrates the ability toconstruct a modular heat exchanger composed of multiple rows of tubesand centers within each core assembly 22, and the use of a bolt-onheader 40 a that is mechanically fastened to a corresponding manifold30, with a gasket 40 b being used to seal the header-to-manifoldinterface. Advantageously, the use of the header 40 a shown in FIG. 9enables existing copper and brass radiator assemblies to be retrofitwith the brazed core assemblies 22 of this invention.

From the above, it can be seen that a particularly advantageous featureof this invention is that a practicable method is provided formanufacturing a modular heat exchanger that utilizes a brazedtube-and-center core construction. The previous inability to incorporatea brazed core construction within a fully modular heat exchanger designis solved by this invention through the avoidance or control of coreshrinkage during brazing, and through the practice of elastomericallysealing the brazed core(s) with the remainder of the assembly in orderto permit removal and/or replacement of one or more of the brazed cores.As a result, a brazed core assembly manufactured according to thisinvention can be readily assembled with the remaining components of aheat exchanger, with proper tube alignment being achieved.

As such, heat exchangers manufactured in accordance with this inventionincorporate advantages of both a modular heat exchanger design and abrazed core construction. For example, the brazed core construction canbe produced with flat tubes and sinusoidal centers to yield an enhancedheat transfer efficiency attributable to these tube and fin designs anda brazed tube-to-fin joint. Consequently, a heat exchanger manufacturedaccording to this invention can be employed in place of a larger priorart heat exchanger employing mechanically-joined tubes and fins.Furthermore, brazed cores manufactured according to this inventionrequire smaller drying ovens and brazing furnaces than monolithic brazedheat exchangers of the prior art, since the brazed core is significantlysmaller than the final assembled heat exchanger unit.

Advantages of this invention associated with its modular constructioninclude a heat exchanger that is more durable for heavy-dutyapplications than heat exchangers having a monolithic brazedconstruction. For example, the use of a modular brazed core resilientlymounted to and elastomerically sealed with the headers of the heatexchanger permits greater flexing without creating a leak path at thetube to header joint. Furthermore, the modular construction enablesreplacement of a damaged brazed core without necessitating replacementor scrappage of the entire unit, such that downtime for the equipment issignificantly reduced. In addition, the modular brazed core assembly ofthis invention can be retrofit on many existing modular heat exchangerunits in the field.

Another advantage of this invention is that the method enables the useof multiple brazed core assemblies within a single heat exchanger.Alternatively, multiple core assemblies can be employed to provide anumber of discrete heat exchanger circuits, while sharing common headersand tanks so as to optimize space and simplify installation.

Finally, the strip grommet design of this invention is economical tomanufacture and install. An additional benefit is that the strip grommetdesign provides galvanic insulation between the brazed core assembly andthe remainder of the heat exchanger. As such, the manifolds and/orheaders can be formed from a metal more noble than the core assemblywithout promoting corrosion due to galvanic action. For example, themanifolds and headers could be formed from a galvanized steel, while thecore assembly is formed from an aluminum alloy. The use of a strongermaterial for the manifolds and headers allows for greater latitude inthe design of these components. The strip grommet design is alsocompatible with oval and rectangular manifold designs known in the priorart. As such a manifold distorts to a more circular shape, the stripgrommet design of this invention is subjected to greater compression,thereby enhancing the sealing capability of the grommet.

Accordingly, while our invention has been described in terms of apreferred embodiment, it is apparent that other forms could be adoptedby one skilled in the art. For example, one or more brazed coreassemblies fabricated according to this invention could be utilized in amodular heat exchanger that differs markedly in construction andappearance from those shown in the Figures, other clad slurrycomposition could be used, and the processing steps could differ fromthose described. Accordingly, the scope of our invention is to belimited only by the following claims.

1. A method for manufacturing a modular heat exchanger, the methodcomprising the steps of: assembling and fixturing a headerless corecomprising substantially parallel tubes separated by centers that extendparallel to the tubes, the tubes and the centers being formed of metalalloys, at least some of the tubes and the centers being clad with abraze alloy that has a melting temperature near but below the meltingtemperature of the metal alloys, each of the tubes having oppositelydisposed open ends and an oblong cross-section definingoppositely-disposed planar surfaces that are spaced from a longitudinalaxis of the tube, at least one of the planar surfaces of each tubecontacting at least one of the centers, the headerless core beingfixtured so that each adjacent pair of the tubes is spaced apart by oneof the centers and spacings between the tubes and spacings between theopen ends of the tubes are determined by the centers so as to establisha pre-brazed tube spacing pattern of the headerless core, the fixturingof the headerless core also minimizing core shrinkage from compaction ofthe tubes and centers during brazing at a brazing temperature of atleast the melting temperature of the braze alloy and at which the tubesand centers thermally expand; brazing the headerless core at the brazingtemperature to yield a brazed headerless core with a post-brazed tubespacing pattern that enables assembly of the tubes with apertures thatare defined in at least one header and correspond to the post-brazedtube spacing pattern of the brazed headerless core, wherein spacingsbetween the open ends of the tubes are not established by any structuralelement that is in addition to the braze alloy and centers and serves asa permanent and integral tube-end alignment fixture of the brazedheaderless core; installing elastomeric sealing means in the aperturesof the header, the elastomeric sealing means having openings thatcorrespond to the post-brazed tube spacing pattern of the brazedheaderless core; and then assembling the brazed headerless core with theheader, wherein an end of each of the tubes is received in acorresponding one of the openings of the elastomeric sealing means toform fluid-tight seals between the tubes and the header, the brazedheaderless core being a removable monolithic brazed module of themodular heat exchanger.
 2. The method according to claim 1, wherein thetubes and the centers are formed of aluminum-based alloys and the brazealloy is an aluminum-based braze alloy.
 3. The method according to claim1, wherein the headerless core is fixtured to permit the headerless coreto expand in response to thermal expansion of the tubes and centersduring the brazing step, such that core shrinkage due to compaction ofthe tubes and centers is sufficiently minimized during the brazing stepso as to enable assembly of the tubes with the openings in theelastomeric sealing means, the fixturing step further maintaining acompression force on the headerless core throughout the brazing step soas to maintain tube to center contact.
 4. The method according to claim1, further comprising the steps of: forming the tubes and centers fromclad aluminum stock; installing spacing members in the headerless coreprior to the brazing step, the spacing members being disposed at theends of the tubes so as to restrict relative movement of the ends duringthe brazing step; and removing the spacing members following the brazingstep and before assembling the brazed headerless core with the header.5. The method according to claim 1, wherein the header is an integralportion of a tank prior to assembling the brazed headerless coretherewith.
 6. The method according to claim 5, wherein the header isformed separately from the tank and then bonded to the tank.
 7. Themethod according to claim 5, wherein the tank is a single-piece tank andthe header is an integral wall thereof and formed therewith.
 8. Themethod according to claim 1, wherein the headerless core is fixturedwith means for applying a variable spring-biased load that enables theheaderless core to be assembled in an oversize condition to compensatefor melting of the braze alloy and softening of the centers during thebrazing step, and enables the headerless core to expand during thebrazing step as a result of thermal expansion of the tubes and centers.9. A method for manufacturing a modular heat exchanger, the methodcomprising the steps of: assembling and fixturing a headerless corecomprising substantially parallel tubes separated by sinusoidal centersthat extend parallel to the tubes, the tubes and the centers beingformed of aluminum-based alloys, at least some of the tubes and thecenters being clad with an aluminum-containing braze alloy that has amelting temperature near but below the melting temperature of thealuminum-based alloys, each of the tubes having oppositely disposed openends and an oblong cross-section defining oppositely-disposed planarsurfaces that are spaced from a longitudinal axis of the tube, at leastone of the planar surfaces of each tube contacting at least one of thecenters, the headerless core being fixtured so that spacing between eachadjacent pair of the tubes and between the open ends thereof beingestablished by the centers so as to establish a pre-brazed tube spacingpattern of the headerless core, the fixturing of the headerless corealso minimizing core shrinkage from compaction of the tubes and centersduring brazing at a brazing temperature of at least the meltingtemperature of the braze alloy and at which the tubes and centersthermally expand; brazing the headerless core at the brazing temperatureso that each the adjacent pairs of the tubes is brazed to acorresponding one of the centers therebetween to form a brazedheaderless core having a post-brazed tube spacing pattern that enablesassembly of the tubes with apertures that are defined in at least oneheader and correspond to the post-brazed tube spacing pattern of thebrazed headerless core, wherein spacings between the tubes and spacingsbetween the open ends of the tubes are not established by any structuralelement that is in addition to the braze alloy and centers and serves asa permanent and integral tube-end alignment fixture of the brazedheaderless core.
 10. The method according to claim 9, wherein theheaderless core is fixtured so as to permit the headerless core toexpand in response to thermal expansion of the tubes and centers duringthe brazing step to inhibit core shrinkage due to compaction of thetubes and centers during the brazing step, the fixturing step furthermaintaining a compression force on the headerless core throughout thebrazing step so as to maintain tube to center contact.
 11. The methodaccording to claim 9, further comprising the steps of: assemblingspacing members to the headerless core, the spacing members beingdisposed at the ends of the tubes so as to restrict relative movement ofthe ends during the brazing step; and then removing the spacing membersfollowing the brazing step.
 12. The method according to claim 9, whereinthe headerless core shrinks during the brazing step as the braze alloymelts.
 13. The method according to claim 9, wherein the pre-brazed tubespacing pattern is substantially the same as the post-brazed tubespacing pattern.
 14. The method according to claim 9, further comprisingthe steps of: providing a tank having the header as an integral wallthereof, the header having elastomeric sealing means in the aperturesthereof with openings that correspond to the post-brazed tube spacingpattern of the headerless core; and then assembling the brazedheaderless core with the tank, wherein an end of each of the tubes isreceived in a corresponding one of the openings of the elastomericsealing means to form fluid-tight seals between the tubes and the tank,the brazed headerless core being a removable monolithic brazed module ofthe modular heat exchanger.
 15. The method according to claim 9, whereinthe headerless core is fixtured with means for applying a variablespring-biased load that enables the headerless core to be assembled inan oversize condition to compensate for melting of the braze alloy andsoftening of the centers during the brazing step, and enables theheaderless core to expand during the brazing step as a result of thermalexpansion of the tubes and centers.
 16. The method according to claim 9,wherein the headerless core is fixtured with at least one movable membersupported in a fixed frame, the movable member being biased against theheaderless core with springs that permit the movable member to bedisplaced toward the fixed frame while generating an increasing load onthe movable member.